1 2 :mod:`marshal` --- Internal Python object serialization 3 ======================================================= 4 5 .. module:: marshal 6 :synopsis: Convert Python objects to streams of bytes and back (with different 7 constraints). 8 9 10 This module contains functions that can read and write Python values in a binary 11 format. The format is specific to Python, but independent of machine 12 architecture issues (e.g., you can write a Python value to a file on a PC, 13 transport the file to a Sun, and read it back there). Details of the format are 14 undocumented on purpose; it may change between Python versions (although it 15 rarely does). [#]_ 16 17 .. index:: 18 module: pickle 19 module: shelve 20 21 This is not a general "persistence" module. For general persistence and 22 transfer of Python objects through RPC calls, see the modules :mod:`pickle` and 23 :mod:`shelve`. The :mod:`marshal` module exists mainly to support reading and 24 writing the "pseudo-compiled" code for Python modules of :file:`.pyc` files. 25 Therefore, the Python maintainers reserve the right to modify the marshal format 26 in backward incompatible ways should the need arise. If you're serializing and 27 de-serializing Python objects, use the :mod:`pickle` module instead -- the 28 performance is comparable, version independence is guaranteed, and pickle 29 supports a substantially wider range of objects than marshal. 30 31 .. warning:: 32 33 The :mod:`marshal` module is not intended to be secure against erroneous or 34 maliciously constructed data. Never unmarshal data received from an 35 untrusted or unauthenticated source. 36 37 .. index:: object; code, code object 38 39 Not all Python object types are supported; in general, only objects whose value 40 is independent from a particular invocation of Python can be written and read by 41 this module. The following types are supported: booleans, integers, long 42 integers, floating point numbers, complex numbers, strings, Unicode objects, 43 tuples, lists, sets, frozensets, dictionaries, and code objects, where it should 44 be understood that tuples, lists, sets, frozensets and dictionaries are only 45 supported as long as the values contained therein are themselves supported; and 46 recursive lists, sets and dictionaries should not be written (they will cause 47 infinite loops). The singletons :const:`None`, :const:`Ellipsis` and 48 :exc:`StopIteration` can also be marshalled and unmarshalled. 49 50 .. warning:: 51 52 On machines where C's ``long int`` type has more than 32 bits (such as the 53 DEC Alpha), it is possible to create plain Python integers that are longer 54 than 32 bits. If such an integer is marshaled and read back in on a machine 55 where C's ``long int`` type has only 32 bits, a Python long integer object 56 is returned instead. While of a different type, the numeric value is the 57 same. (This behavior is new in Python 2.2. In earlier versions, all but the 58 least-significant 32 bits of the value were lost, and a warning message was 59 printed.) 60 61 There are functions that read/write files as well as functions operating on 62 strings. 63 64 The module defines these functions: 65 66 67 .. function:: dump(value, file[, version]) 68 69 Write the value on the open file. The value must be a supported type. The 70 file must be an open file object such as ``sys.stdout`` or returned by 71 :func:`open` or :func:`os.popen`. It may not be a wrapper such as 72 TemporaryFile on Windows. It must be opened in binary mode (``'wb'`` 73 or ``'w+b'``). 74 75 If the value has (or contains an object that has) an unsupported type, a 76 :exc:`ValueError` exception is raised --- but garbage data will also be written 77 to the file. The object will not be properly read back by :func:`load`. 78 79 .. versionadded:: 2.4 80 The *version* argument indicates the data format that ``dump`` should use 81 (see below). 82 83 84 .. function:: load(file) 85 86 Read one value from the open file and return it. If no valid value is read 87 (e.g. because the data has a different Python version's incompatible marshal 88 format), raise :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. The 89 file must be an open file object opened in binary mode (``'rb'`` or 90 ``'r+b'``). 91 92 .. note:: 93 94 If an object containing an unsupported type was marshalled with :func:`dump`, 95 :func:`load` will substitute ``None`` for the unmarshallable type. 96 97 98 .. function:: dumps(value[, version]) 99 100 Return the string that would be written to a file by ``dump(value, file)``. The 101 value must be a supported type. Raise a :exc:`ValueError` exception if value 102 has (or contains an object that has) an unsupported type. 103 104 .. versionadded:: 2.4 105 The *version* argument indicates the data format that ``dumps`` should use 106 (see below). 107 108 109 .. function:: loads(string) 110 111 Convert the string to a value. If no valid value is found, raise 112 :exc:`EOFError`, :exc:`ValueError` or :exc:`TypeError`. Extra characters in the 113 string are ignored. 114 115 116 In addition, the following constants are defined: 117 118 .. data:: version 119 120 Indicates the format that the module uses. Version 0 is the historical format, 121 version 1 (added in Python 2.4) shares interned strings and version 2 (added in 122 Python 2.5) uses a binary format for floating point numbers. The current version 123 is 2. 124 125 .. versionadded:: 2.4 126 127 128 .. rubric:: Footnotes 129 130 .. [#] The name of this module stems from a bit of terminology used by the designers of 131 Modula-3 (amongst others), who use the term "marshalling" for shipping of data 132 around in a self-contained form. Strictly speaking, "to marshal" means to 133 convert some data from internal to external form (in an RPC buffer for instance) 134 and "unmarshalling" for the reverse process. 135 136
